Experiment 5

Separation of chloride and bromide by ion chromatography

What is Ion Chromatography?

Most of the chemical analysis required separation. The separation can be referring to the isolation of targeted analytes from sample matrices or other analytes. Chromatography is an analytical technique commonly used for separating a mixture of chemical substances into its individual components. To get the separation process started, the mixture is dissolved in a substance called the mobile phase (liquid or gas), which carries it through a second substance called the stationary phase.

This experiment will introduce the basic concept of chromatography. You will observe how the ions can be separated from each other using ion chromatography. Ion chromatography is used for water chemistry analysis. Ion chromatographs are able to measure concentrations of major anions, such as fluoride, chloride, nitrate, nitrite, and sulfate, as well as major cations such as lithium, sodium, ammonium, potassium, calcium, and magnesium. Concentrations of organic acids can also be measured through ion chromatography.

How Does Ion Chromatography Work?

Ion chromatography, a form of liquid chromatography, measures concentrations of ionic species by separating them based on their interaction with a resin. Ionic species separate differently depending on species type and size. Sample solutions pass through a chromatographic column where ions are absorbed by column constituents. As an ion extraction liquid, known as eluent, runs through the column, the absorbed ions begin separating from the column. The retention time of different species determines the ionic concentrations in the sample.

Separation of ions via ion exchange column chromatography.

Column Chromatography

Interaction between ions and stationary phase.

The anion exchange resin used in this experiment (Amberlite IRA-900 Chloride) is a cross-linked polymer containing quaternary ammonium groups as integral parts of the polymer lattice and an equivalent amount of chloride anions. The anion exchange resin, originally in the chloride form, is converted into the nitrate form by washing with sodium nitrate solution.

The function of the ion exchange resin depends on the following chemical equilibrium:

Resin-Cl- + NO₃- --> Resin-NO₃- + Cl- (Equation 5.1)

The above equation shows that concentrated nitrate ion will shift the equilibrium to the right and chloride ion will be eluted from the column slowly. During the process, there is a difference in concentration of nitrate and chloride ions, hence equilibrium exists along the length of the column. The process of elution should be allowed to run slowly to attain equilibrium stability. Ion exchange procedure is used widely in synthesis and analysis. One important usage is in the separation of the actinide and lanthanide elements.

In this experiment, a mixture of chloride and bromide ions will be separated quantitatively. These two anions exchange readily with the resin-nitrate, i.e. equilibrium shifts to the left in (Eq 5-1) when the solution mixture is poured into the column. The anions are eluted from the column when a solution of sodium nitrate is passed through the column. Separation is possible as bromide ion is adsorbed stronger than the chloride ion (Keq for Br < Keq for Cl). The progress of separation is followed by titrating 10 cm³ fraction of the eluate with standard silver nitrate solution.

This titration uses chromate ion (CrO₄^2-) as an indicator. Low concentration of the indicator is needed to achieve its end point when excess Ag⁺ is added. Blank titration is done to determine the actual volume of silver nitrate needed to form Ag₂CrO₄ and to determine the end point. During titration, Ag⁺reacts halides to form a white precipitate. After the halides are completely consumed, the additional Ag⁺will react with CrO₄^2-to form a red precipitate which indicates the end point.

Methodology

Preparation of Column

Wash about 20 g of resin with distilled water in a beaker for several minutes. Any fine particles are removed by decantation, and the washing procedure is repeated several times until the color of the decanted washing is clear.
Wash the resin with dilute HNO₃ until the washings are free from chloride ions (silver nitrate test).
Transfer the resin slurry portion-wise into a column that has a glass-wool plug at the lower end and is filled with water. (The tube may be tapped gently to prevent the formation of air bubbles).
Fill a 250 cm³ separating funnel with 0.30 mol dm^-3 NaNO₃ and elute HNO₃ from the column for 15 minutes.

(To obtain a satisfactory separation, it is essential that the solutions should pass through the column in a uniform manner. The resin particles should be packed uniformly in the column: the resin bed should be free from air bubbles so that there is no channeling)

Blank

Before commencing the elution, titrate 10.0 cm³ of the 0.30 mol dm^-3 sodium nitrate (with 2 drops of potassium chromate as an indicator) with the 0.05 M silver nitrate solution that has been diluted ten times (0.005M) and retain the product of the blank titration for comparing with the color in the actual titrations of the eluates. Color change from yellow to red-brown. (Do not forget to change the volume to the original concentration for determination of chloride and bromide). This experiment will enable the determination of the volume silver nitrate used to react with the indicator.

Determination of chloride and bromide

Mixture for separation: Weigh out accurately about 0.1010 g of sodium chloride, and about 0.2037 g of potassium bromide. Dissolve in about 2.0 cm³ of water and transfer quantitatively to the top of the column with the aid of 0.30 mol dm^-3 sodium nitrate.
Pass 0.30 mol dm^-3 sodium nitrate through the column at a flow rate of about 1 cm³ per minute and collect the effluent in 10 cm³ fractions using a 10 cm³ measuring cylinder.
Transfer each fraction in turn to a conical flask and add 2 drops of potassium chromate solution as an indicator and titrate with standard 0.05 mol dm^-3 silver nitrate.
Plot a graph of the total effluent collected against the concentration of halide in each fraction (millimoles per liter). From the graph, calculate the percentage of chloride ion and bromide ion recovered.

Note:

The titer falls to zero after all the chloride ion has been eluted and increases as the bromide ion is eluted from the column. If the titer does not fall exactly to zero, adjustment to the plot has to be made. Do not forget to deduct the blank volume for each titration.

Raw Data

Determination of blank:

Table 1: Volume of AgNO₃used in the blank determination.

Determination of chloride and bromide

Table 2: Volume of AgNO₃used in the chloride and bromide determination.

Task:

Calculate the volume of AgNO₃used by the indicator based on the data in Table 1.
Based on Table 2, calculate the concentration of halide(s) for each collected effluent. Do not forget to deduct the blank volume for each titration.
Plot the graph of the concentration of halides versus the volume of total effluent collected.
Calculate the percentage of Cl- and Br- that recovered from the separation.

Question:

What is the role of chromatography in chemical analysis?
Name the stationary phase and mobile phase used in this experiment.
What is the meaning of ion-exchange in this study?

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